Ethylene cone-shell polymer powders

ABSTRACT

A catalyst system for the polymerization of alpha-olefins comprising a chromium-containing catalyst and an yttrium-containing catalyst wherein the yttrium-containing catalyst is represented by the formula (Cp 2  YX x ) y  ·M z  L n , wherein Cp is cyclopentadienyl or cyclopentadienyl substituted with an alkyl or alkyl silyl radical, X is a halogen, M is an alkali metal, L is a suitable electron donor ligand, x is 1 or 2, y is 1 or 2, z is 0 or 1, and n is a number corresponding to the value needed to form a stable complex, with the proviso that when y is 2, z and n are 0 is provided. Also disclosed is a polymerization process employing such a catalyst system, and novel olefin polymers. Polymers thus produced exhibit high bulk density and a broad molecular weight distribution.

This is a divisional of application Ser. No. 08/153,694 filed Nov. 17,1993 now U.S. Pat. No. 5,399,622.

FIELD OF THE INVENTION

The present invention relates to the polymerization of olefins. In aparticular aspect, the present invention relates to olefinpolymerization employing a catalyst system comprising anyttrium-containing catalyst and a chromium-containing catalyst.

BACKGROUND OF THE INVENTION

Various techniques have been employed in the past for the polymerizationof polymers and copolymers of olefins. One approach has involvedemploying catalysts containing chromium. Typically such polymerizationsare carried out at relatively low temperatures and pressures.

For many applications, such as extrusion and molding processes, it ishighly desirable to have polymers which have a broad molecular weightdistribution. Such polymers exhibit excellent processability, i.e., theycan be processed at a faster throughput rate with lower energyrequirements with reduced melt flow perturbations.

Some techniques for preparing such polymers have involved the use ofmultiple reactor arrangements. Such multiple reactor schemes, whileoffering versatility in resin characteristics, can be less efficientthan would be desired. Preparing the polymer in a single reactor wouldbe much more economical.

It is also desirable to obtain a bimodal, or broad molecular weightdistribution ethylene polymer in which comonomer is incorporated intothe high molecular weight portion of the molecular weight distribution.Such polymers exhibit good impact resistance, tensile strength,elongation, flexural modulus, and environmental stress crack resistance.

In commercial applications, high bulk density is important for practicalconsiderations such as convenient transfer and handling of the polymer.It is also important in commercial applications to be able to produce awide spectrum of polymers so far as melt flow is concerned, i.e.molecular weight.

It would therefore be desirable to provide a catalyst capable ofpreparing polymers having high bulk density, broad molecular weightdistribution with comonomer incorporated in the high molecular weightportion, while employing a single reactor.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a catalyst systemuseful for the polymerization of olefins having broad molecular weightdistribution in a single reactor.

It is another object of this invention to provide a polymerizationprocess for preparing polymers having a broad molecular weightdistribution which have branching in the high molecular weight portion.

It is another object of this invention to provide a polymer having highbulk density.

It is another object of this invention to provide a polymerizationprocess for producing polymers having a wide range of molecular weights.

In accordance with the present invention, there is provided a catalystsystem for the polymerization of alpha-olefins comprising achromium-containing catalyst and an yttrium-containing catalyst, whereinthe yttrium-containing catalyst is represented by the formula (Cp₂YX_(x))_(y).M_(z) L_(n), wherein Cp is cyclopentadienyl orcyclopentadienyl substituted with an alkyl or alkyl silyl radical, X isa halogen, M is an alkali metal, L is a suitable electron donor ligand,x is 1 or 2, y is 1 or 2, z is 0 or 1, and n is a number correspondingto the value needed to form a stable complex, with the proviso that wheny is 2, z and n are 0.

Other aspects of the present invention, are to provide a process forpreparing such catalyst systems, a polymerization process employing suchcatalyst systems, and the olefin polymers thus produced.

DETAILED DESCRIPTION OF THE INVENTION

The particular chromium-containing catalyst employed can be readilyselected by one skilled in the art after having the benefit of thisdisclosure.

A large number of patents exist describing various suitablechromium-containing catalysts. Some examples include U.S. Pat. Nos.3,887,494; 3,900,457; 4,053,436; 4,081,407; 4,101,722; 4,150,208;4,151,122; 4,294,724; 4,296,001; 4,345,055; 4,364,839; 4,364,842;4,364,854; 4,392,990; 4,397,765; 4,402,864; 4,405,501; 4,735,931; and4,966,951; the disclosures of which are incorporated herein byreference.

Generally the chromium-containing catalyst is a chromium oxide.Preferably, a chromium-containing catalyst is selected that is notparticularly affected by hydrogen or by the small amounts of thecocatalyst that is used with the yttrium-containing catalyst. It is alsopreferred that the chromium-containing catalyst be more effective inincorporating comonomer than the yttrium-containing catalyst under theselected polymerization conditions, thus producing a polymer having arelatively low density, i.e. a density in the range of from about 0.88g/cc to about 0.93 g/cc measured according to ASTM D 1505-68.

The amount of chromium in the chromium-containing catalyst can vary overa wide range. Any suitable catalytic amount can be employed. Typically,the chromium-containing catalyst will contain about 0.1 to about 10weight percent chromium based on the weight of the chromium-containingcatalyst, more preferably about 0.1 to about 5 weight percent chromium.

The currently preferred chromium-containing catalysts are silicasupported catalysts prepared by incorporating chromium on silica. Morepreferably, the chromium-containing catalyst further comprises titania.

Generally, the catalysts can be activated in air at a temperature in therange of about 300° C. to about 1000° C., preferably about 300° C. toabout 800° C. Generally the activation is carried out for about one halfto about 50 hours, preferably about 2 to about 10 hours.

Optionally, the chromium-containing silica can then be reduced withcarbon monoxide at a temperature in the range of about 300° C. to about500° C., preferably about 300° C. to about 450° C. Catalysts suitablefor such an activation and reduction process include catalysts which arecommercially available from W. R. Grace, Davison Catalyst Division,under the trade designations 963, 967 MS, 968 MS, 969 1D, and 969 MS.

The particular yttrium-containing catalyst to be employed can readily bedetermined by those skilled in the art after having had the benefit ofthis disclosure.

Such catalysts, for example, are disclosed in U.S. Pat. Nos. 5,066,739;5,232,999; and 5,244,991, the disclosures of which are incorporatedherein by reference. Preferably, an yttrium-containing catalyst isselected that is sensitive to the presence of hydrogen.

The yttrium-containing catalysts are represented by the formula (Cp₂YX_(x))_(y).M_(z) L_(n), wherein Cp is cyclopentadienyl orcyclopentadienyl substituted with an alkyl or alkyl silyl radical, X isa halogen, M is an alkali metal, L is a suitable electron donor ligand,x is 1 or 2, y is 1 or 2, z is 0 or 1, and n is a number correspondingto the value needed to form a stable complex, with the proviso that wheny is 2, z and n are 0.

Generally, the substituents of the substituted cyclopentadienyl wouldeach have 1 to 4 carbon atoms. Some specific examples of the Cp groupsinclude cyclopentadienyl, ethylcyclopentadienyl,trimethylcyclopentadienyl, trimethylsilylcyclopentadienyl, andpentamethylcyclopentadienyl. Pentamethylcyclopentadienyl is preferred.The formula H₅ C₅ is used herein to refer to cyclopentadienyl. Theformula Me₅ C₅ is used herein to refer to pentamethylcyclopentadienyl.

The halogens, X, of the above formula are preferably chloride or iodide,more preferably chloride.

Generally M is selected from lithium, sodium and potassium. Lithium andpotassium are preferred.

The L of the above formula can be selected from any suitable electrondonor ligand. Some specific examples of electron donor ligands includetetrahydrofuran, diethyl ether, and N,N,N',N'-tetramethylethylenediamine. Tetrahydrofuran (THF) is preferred.

Examples of specific yttrium-containing catalysts include

(Me₅ C₅)₂ YCl₂.K(THF)_(n)

(H₅ C₅)₂ YCl₂.K(THF)_(n)

(Me₅ C₅)₂ YCl₂.Li(THF)_(n)

(H₅ C₅)₂ YCl₂.Li(THF)_(n)

((Me₅ C₅)₂ YCl₂

((H₅ C₅)₂ YCl₂

(Me₅ C₅)₂ YCl.THF

(H₅ C₅)₂ YCl.THF

The compounds Cp₂ YX₂.ML_(n) and Cp₂ YX.L_(n) can be prepared byrefluxing the electron donor ligand, an alkali metal hydride, andcyclopentadiene or a substituted cyclopentadienyl compound to obtainMCp.L_(n). Yttrium trihalide, MCp.L_(n), and the electron donor ligandare then reacted. Purified Cp₂ YX₂.ML_(n) and Cp₂ YX.L_(n) can beobtained by crystallizing the resulting reaction mixture. A typicalliquid used for such crystallization could be toluene. The toluenesoluble portion yields Cp₂ YX.L_(n) and the toluene insoluble portionyields Cp₂ YX₂.ML_(n).

For example, one method for preparing the yttrium-containing catalystsCp₂ YX₂.K(THF)_(n) and Cp₂ YX.THF is to react pentamethylcyclopentadienewith potassium hydride by refluxing with tetrahydrofuran. Thecyclopentadienyl potassium compound resulting from this step is thenreacted with yttrium trihalide. The molar ratio of the cyclopentadienylpotassium compound to the yttrium trihalide compound is about 2 to 1.This reaction can be carried out by refluxing in THF.

The compound (Cp₂ YX)₂ can be prepared by subjecting solid Cp₂ YX.L tosublimation under suitable conditions. It is generally found that thesublimation is adequately carried out at a temperature of about 230° C.when the electron donor ligand is THF.

The weight ratio of the yttrium-containing catalyst tochromium-containing catalyst can vary broadly depending upon theparticular properties desired. As a general rule, the weight ratio ofthe yttrium-containing catalyst to chromium-containing catalyst would bein the range of from about 0.01:1 to about 100:1, preferably in therange of from about 0.1:1 to about 50:1, and more preferably in therange of from 0.1:1 to 10:1.

Typically the total catalyst system would be present in the range offrom about 0.001 weight pet-cent to about 1 weight percent based on thetotal weight of the polymerization reaction mixture.

Generally, it is advantageous to use the yttrium-containing catalyst incombination with a small but effective activating amount of an alkalimetal alkyl or alkaline earth metal alkyl cocatalyst. Any suitablealkali metal alkyl or alkaline earth metal alkyl can be employed as thecocatalyst. Generally alkyllithiums are preferred. Generally the alkylradicals of the cocatalyst would contain 1 to 12 carbon atoms,preferably 1 to 6 carbon atoms. Some specific examples of suchcocatalysts include n-butylsodium, n-butyllithium,secondary-butyllithium, tertiary-butyllithium, n-butylpotassium,diethylmagnesium, di-n-butylmagnesium, and combinations thereof.

The ratio of the the cocatalyst to yttrium-containing catalyst can varyover a wide range depending upon the particular compounds employed andthe particular results desired. As a general rule the molar ratio of themetal in the cocatalyst to the yttrium in the yttrium-containingcatalyst will be in the range of from about 0.5:1 to about 20:1,preferably about 1:1 to about 15:1.

The present catalyst system is effective in polymerizing a wide range ofolefins, particularly aliphatic alpha-olefins having 2 to about 18carbon atoms, preferably 2 to 8 carbon atoms. The term polymerization isused herein to include both homo- and co-polymerization. Some examplesof suitable olefins include ethylene, propylene, butene-1, hexene-1,octene-1, 4-methyl-1-pentene, and mixtures thereof.

The present catalyst system is particularly useful for thepolymerization of ethylene in combination with small amounts of higheralpha olefin comonomer, such as butene-1 or hexene-1. Generally thecomonomer is present in amounts of less than 20 weight % based on theweight of the ethylene. It is also within the scope of the invention toemploy a chromium-containing catalyst capable of producing comonomerin-situ. Preferably ethylene polymers contain at least 90 mole percentethylene, and more preferably at least 95 mole percent ethylene.

The polymerizations can be carried out in a slurry type process.Typically, this requires the employment of polymerization temperaturesin the range of from about 0° C. to about 170° C., more preferably about10° C. to about 100° C. The polymerization pressures are generally inthe range of from about 100 psia to about 700 psia, or higher.

The reaction can be conducted in a batch reactor or in a suitablecontinuous reactor. It is generally preferable to carry out thepolymerization in an elongated reaction tube which is contactedexternally with suitable cooling media to maintain the desiredpolymerization temperature. A preferred technique uses a loop reactor inwhich the reaction mixture and polymer is circulated within a pipe loop.The time involved for the polymerization will vary depending upon theparticular catalyst mixture employed, the temperature, and the desiredtype of polymer. Typically, when the polymerization is conducted on acommercial scale the residence time is in the range of about one halfhour to about two hours.

It is generally desirable to carry out the polymerization in the absenceof moisture and oxygen. As a general rule the polymerization isconducted in the presence of a suitable liquid diluent. Examples of suchdiluents include isobutane, n-butane, n-hexane, isooctane, cyclohexane,methylcyclopentane, dimethylcyclohexane, and combinations thereof.

In order to obtain polymers with particular physical properties it maybe desirable to employ hydrogen during the polymerization period.Hydrogen, when employed, can vary over a broad range depending upon theparticular compounds employed as the catalyst and the particular resultsdesired. Typically, the hydrogen would be employed at a pressure in therange of from about 10 psi to about 200 psi, more preferably in therange of from about 15 psi to about 100 psi.

The monomers can be contacted with the yttrium-containing catalyst andthe chromium-containing catalyst in any sequence desired. In a preferredembodiment, the monomers are contacted with the chromium-containingcatalyst under polymerization conditions prior to contacting with theyttrium-containing catalyst.

In another embodiment, a polymer composite powder is provided comprisinga low density ethylene polymer core surrounded by a high densityethylene polymer coating. The low density ethylene polymer core willgenerally have a density in the range of from about 0.88 g/cc to about0.94 g/cc measured according to ASTM D 1505-68, preferably in the rangeof from about 0.90 g/cc to about 0.94 g/cc. The high density ethylenepolymer coating will generally have a density in the range of fromgreater than about 0.94 g/cc to about 0.98 g/cc measured according toASTM D 1505-68, preferably in the range of from about 0.95 g/cc to about0. 98 g/cc.

The amounts of low density ethylene polymer core and high densityethylene polymer coating can vary broadly. Generally the low densityethylene polymer core will be present in an amount in the range of fromabout 10 weight percent to about 90 weight percent based on the totalweight of the polymer composite powder, preferably in the range of fromabout 25 weight percent to about 90 weight percent, and more preferablyin the range of from 50 weight percent to 90 weight percent. Generallythe high density ethylene polymer coating will be present in an amountin the range of from about 90 weight percent to about 10 weight percentbased on the total weight of the polymer composite powder, preferably inthe range of from about 75 weight percent to about 10 weight percent,and more preferably in the range of from 50 weight percent to 10 weightpercent.

A further understanding of the present invention and its various aspectsand advantages will be provided by the following examples. The followingexamples will serve to show the present invention in detail by way ofillustration and not by way of limitation.

EXAMPLES

Examples IA and IB demonstrate the polymerization and properties ofpolymers prepared employing an yttrium-containing catalyst.

Example II demonstrates the polymerization and properties of polymersprepared employing a chromium-containing catalyst.

Example III demonstrates the polymerization and properties of a polymercomposite prepared by first preparing ethylene copolymer with achromium-containing catalyst and then polymerizing ethylene in thepresence of the ethylene copolymer and an yttrium-containing catalyst.

Example IV demonstrates the polymerization and properties of polymersprepared employing a catalyst system comprising a chromium-containingcatalyst and an yttrium-containing catalyst.

Example V was conducted on a pilot plant scale and demonstrates thepolymerization and properties of polymers prepared employing achromium-containing catalyst, an yttrium-containing catalyst, and acatalyst system comprising a combination of the two catalysts.

Example IA

In Example IA, the yttrium-containing catalyst Cp₂ YCl₂ K(THF)_(n) wasemployed to polymerize ethylene.

The yttrium-containing catalyst employed in Run 101 was prepared in thefollowing manner. A slurry of 100 mL tetrahydrofuran (THF) and 2.56 g KH(0.064 moles) was prepared in a flask. To the slurry, 10 mL ofpentamethylcyclopentadiene (0.064 moles Cp) were added by means of asyringe. The slurry was refluxed for 48 hours and then allowed to cool.The slurry was filtered separating a deep red solution and colorlesssolid. The colorless solid was washed with 2×25 mL THF. Excess THF wasremoved from the solid material under vacuum and 13.5 g KCp.THF_(n) wasrecovered.

To a round bottom, 500 mL flask, 2.00 g yttrium chloride (0.0102 molesYCl₃), was cautiously added to 100 mL of tetrahydrofuran (THF) withrapid stirring. In a dry box, 5.05 g of KCp.THF_(n) (0.0205 moles),prepared as described above, was added to the reaction mixture over aperiod of 10 minutes. The light tan reaction mixture was stirred overnight. The reaction mixture was filtered to remove residual solids. THFwas removed from the filtrate in vacuum. The dried solids were extractedwith 2×30 mL toluene. The toluene insolubles yielded 4.23 g Cp₂ YCl₂K(THF)_(n). The toluene solubles yielded 0.63 g Cp₂ YCl.THF.

Polymerization was conducted in a four liter autoclave equipped withstirrer (1200 RPM) and jacket. The autoclave was charged with 0.0435 gCp₂ YCl₂ K(THF)_(n), the toluene insoluble fraction described above,0.89 mL of 0.16M n-butyllithium in hexane as cocatalyst, and 80 g ofn-hexene-1 comonomer. The catalyst to cocatalyst molar ratio was 1:2 inthe reactor. Two liters of isobutane diluent was then added and thereactor was pressurized with ethylene to yield a total pressure of 300psig. Steam and water were introduced into the jacket to maintain atemperature of 90° C. After one hour of stirring the autoclave wasopened and polymer was recovered. The polymer fluff was extremely fine,mostly smaller than 50μ (passes through a 325 mesh screen), and tendedto have a static charge which made recovery difficult. The polymercoated the sides of the autoclave and produced a sticky film which wasdifficult to remove. The polymer also retained liquid isobutane diluent.Wet polymer fluff weighing 485 g, yielded 178 g after drying in an ovenat 80° C. Total activity was 4081 g polymer/g catalyst.hour. The bulkdensity of the polymer produced was about 5 lbs/ft³. The densitymeasured according to ASTM D 1505-68 was 0.9751 g/cc, which indicatedthat no hexene was incorporated. The melt index measured according toASTM D1238-65ST, condition E, was too high to be measured, i.e. >500g/10 min. The polymer was analyzed by size exclusion chromatography. Theresults are summarized in Table I

Example IB

In Example IB, the yttrium-containing catalyst Cp2YCl.THF was employedto polymerize ethylene.

The polymerizations in Runs 102-107 were conducted similar to Run 101,with the exception of employing the yttrium-containing catalystCp2YCl.THF as catalyst and varying the temperature and amount of H₂employed. Also, no hexene was employed in Example IB. The Cp2YCl.THF wasprepared as described above in Example 1A, except the toluene solublefraction was recovered and employed as the catalyst. The reactionconditions and results are summarized in Table I. Mw as used herein isweight average molecular and Mn is number average molecular weight. Thebulk density of the polymer produced was about 5 lbs/ft³.

                  TABLE I                                                         ______________________________________                                                    Temp.   H.sub.2                                                   Run  Cat.   °C.                                                                            psig  IV  Mw/1000                                                                              Mn/1000                                                                              Mw/Mn                             ______________________________________                                        101  IA     90      0     --  16.6   7.2    2.3                               102  IB     90      0     3.9 too high to measure                             103  IB     90      5     0.7 11.9   2.5    4.7                               104  IB     90      100   0.6 6.4    1.2    5.5                               105  IB     25      0     7.2 too high to measure                             106  IB     25      5     0.5 6.2    1.9    3.3                               107  IB     25      15    0.5 1.7    1.0    1.8                               ______________________________________                                         -- did not determine                                                     

It can be seen from Table I, that the polymerizations were sensitive tohydrogen and reaction temperature, which can thus be used to control themolecular weight of the polymer. The results also show that the polymersexhibited a relatively narrow molecular weight distribution, i.e. Mw/Mnof 1.8 to 5.5.

Example II

In Example II, polymerization runs were conducted employing achromium-containing catalyst.

A commercial chromium-containing catalyst, Grade 963 Magnapore,purchased from W. R. Grace, Davison Catalyst Division, was employed. Thecatalyst was a chromium/silica-titania tergel prepared to contain 2.5%Ti. Prior to use, the chromium-containing catalyst was calcined in dryair at 870° C. for three hours and then exposed to carbon monoxide forone half hour at 350° C. Polymerization pressure was 450 psig. Thepolymerization reaction conditions and results are summarized below. MIas used herein is melt index in g/10 min. measured according to ASTMD1238-65T, condition E. HLMI as used herein is high load melt index ing/10 min. measured according to ASTM D1238-65T, condition F. Density wasmeasured in g/cc according to ASTM D 1505-68.

                  TABLE II                                                        ______________________________________                                                    Run                                                                           201      202      203                                             ______________________________________                                        Temperature (°C.)                                                                    90         85       90                                          Run Time (min.)                                                                             30         35       60                                          Wt. Cr cat. (g)                                                                             0.50       0.24     0.27                                        Polymer (g)   308        237      607                                         Productivity  1235       1700     2250                                        (g/g cat.)                                                                    MI (g/10 min.)                                                                              1.66       0.08     1.24                                        HLMI (g/10 min.)                                                                            93.8       10.3     71.0                                        Density (g/cc)                                                                              0.932      0.938    0.926                                       Cocatalyst                                                                    (mL 0.16 M BuLi)                                                                            6.00       2.89     3.24                                        Cocat/cat.    10:1       10:1     10:1                                        Mw            --         --       14,500                                      Mn            --         --        2,700                                      Mw/Mn         --         --       5.4                                         ______________________________________                                         -- did not determine                                                     

Polymer density in the range of 0.926 to 0.938, indicated branching andin-situ hexene production.

Example III

In Example III, ethylene copolymer was prepared employing achromium-containing catalyst. The ethylene polymer was then contactedwith the yttrium-containing catalyst of Example IA and ethylene toproduce a composite polymer.

Ethylene copolymer was prepared by copolymerizing ethylene and 1-hexeneemploying the chromium-containing catalyst described in Example II, at atemperature of 90° C. and a pressure of 550 psig. The ethylene copolymerwas recovered from the reactor and screened so that only coarseparticles remained, i.e. larger than 20 mesh. The ethylene copolymerexhibited a high load melt index, HLMI, of 19 g/10 min. measuredaccording to ASTM D1238-65T, condition F, a melt index, MI, of 0.20 g/10min. measured according to ASTM D1238-65T, condition E, and a density of0.923, measured according to ASTM D 1505-68.

A 200 g sample of the ethylene copolymer was introduced into thereactor. The reactor was degassed by pressurizing the reactor withnitrogen several times. Then 0.0222 g of Cp₂ YCl.THF catalyst preparedas described in Example IB was added followed by 0.59 ml of butyllithiumsolution (1.6 molar). Two liters of isobutane were then added, followedby ethylene to yield a total pressure of 530 psig. The reactortemperature was raised to 90° C. and held for one hour. The reactor wascooled and the composite polymer recovered.

The recovered composite polymer retained the coarse particle size of theoriginal ethylene copolymer and thus provided easy recovery andhandling. The polymer yield was about 240 g. The yttrium-containingcatalyst produced additional polymer at a productivity of about 2000 g/gcatalyst. No fine polymer was found, which indicated that the polymerprepared by the yttrium-containing catalyst coated the polymer preparedby the chromium-containing catalyst. The composite polymer particlesexhibited a melt index of 0.06 g/10 min., which indicated that very highmolecular weight polymer produced by the yttrium-containing catalyst hadcoated the ethylene copolymer.

Example IV

Polymerization runs were conducted employing the yttrium-containingcatalyst (Cp₂ YCl)₂ and the chromium-containing catalyst prepared asdescribed in Example II.

The yttrium-containing catalyst, (Cp₂ YCl)₂, was prepared by refluxingyttrium trichloride and pentamethylcyclopentadienyl sodium in THF andsubjecting the resulting solid to sublimation at 230° C. The molar ratioof the yttrium trichloride to the pentamethylcyclopentadienyl sodium was1:2. The reaction conditions and results are summarized in Table IV.

                  TABLE IV                                                        ______________________________________                                                  Run                                                                          401     402      403      404                                        ______________________________________                                        Temperature (°C.)                                                                  90       90       90     90                                       Time (min.) 60       60       60     25                                       wt. (YCp.sub.2 Cl).sub.2 (g)                                                              0.0458   0.026    0.035  0.030                                    wt. Cr/Si.Ti (g)                                                                          0.0293   0.22     0.26   0.24                                     Cocat.      3.58     0.69     2.40*  0.8                                      mL n-BuLi                                                                     Cocat./Cr   10:1     10:1     10:1   10:1                                     Polymer     189      200      98     364                                      recovered (g)                                                                 Productivity                                                                              2513     815      332    3233                                     (g/g cat.)                                                                    MI g/10 min.                                                                              0.01     0        0      0.03                                     HLMI g/10 min.                                                                            0.81     2.49     0.31   5.8                                      Intrinsic viscosity                                                                       3.38     5.82     8.25   3.60                                     Density (g/cc)                                                                            --       --       --     0.953**                                  ______________________________________                                         -- did not determine                                                          *0.091 M TEA used as cocatlyst                                                ** 20 mL hexene added                                                    

The low MI and HLMI values and high intrinsic viscosity indicated highmolecular weight polymers were produced compared to polymer preparedwith chromium-con, raining catalyst only. The density was also highcompared to polymer prepared with chromium-containing catalyst only.These results indicated a strong contribution from theyttrium-containing polymer. The polymer was easy to handle and did notstick to the walls or the reactor.

Example V

Yttrium-containing catalyst Cp₂ YCl₂ Li.THF_(n), was prepared in a 500mL round bottom flask. To the flask, 200 mL THF was added. The flask wasplaced in an ice bath and 10.0 g YCl₃ was slowly added while stirring.The mixture became cloudy with a slight greyish color. Then 14.56 gpentamethylcyclopentadiene lithium (LiCp) was added. The flask wasremoved from the ice bath and heated to reflux and maintained for 5hours. The liquid was filtered and solvent was stripped from thefiltrate. The yield was about 30 g solid Cp₂ YCl₂ Li.THF_(n).

A commercial chromium-containing catalyst, Grade 963 Magnapore,purchased from W. R. Grace, Davison Catalyst Division, was employed asthe chromium-containing catalyst. The catalyst was achromium/silica-titania tergel prepared to contain 2.5% Ti. Prior touse, the chromium-containing catalyst was calcined in dry air at 649° C.for three hours.

Polymerization runs were conducted employing an 87 liter (23 gallon),15.2 cm diameter pipe loop reactor. Two catalyst feeding vessels wereemployed with separate catalyst feeders for independent control of thecatalysts.

In Run 503, polymerization was initiated and stabilized with thechromium-containing catalyst. Butyllithium was introduced into thereactor during the chromium-containing catalyst polymerization.Approximately 8 hours later, yttrium-containing catalyst was introducedinto the reaction. The chromium-containing catalyst toyttrium-containing catalyst ratio was approximately 2:1 by volume. Thereaction conditions and results are summarized below.

                  TABLE V                                                         ______________________________________                                                    Run                                                                           501      502     503                                              ______________________________________                                        Temperature (°C.)                                                                    89         88      87                                           BuLi (ppm)    4          8       4                                            Hexene-1      0          7       7                                            (wt. % of ethylene)                                                           MI (g/10 min.)                                                                              10.1       0       0.37                                         HLMI (g/10 min.)                                                                            --         1.7     25                                           IV            *          3.66    2.26                                         Density (g/cc)                                                                              --         0.940   0.948                                        Fluff Bulk Density                                                                          2.5        20.9    21.3                                         (lbs/ft.sup.3)                                                                Mw/Mn         --         --      14                                           ______________________________________                                         -- did not determine                                                          *too high to measure                                                     

The ethylene copolymer prepared employing the catalyst system, Run 503,exhibited unexpectedly high bulk density and broad molecular weightdistribution.

That which is claimed is:
 1. A polymer composite powder comprising a lowdensity ethylene polymer core and a high density ethylene polymercoating:wherein said polymer composite powder is produced according tothe process comprising contacting ethylene under polymerizationconditions with a chromium-containing catalyst to produce said lowdensity ethylene polymer core and then contacting said low densityethylene polymer core with ethylene and an yttrium-containing catalystto produce said high density ethylene polymer coating:wherein said lowdensity ethylene polymer core exhibits a density in the range of fromabout 0.90 g/cc to about 0.94 g/cc measured according to ASTM D 1505-68;wherein said high density ethylene polymer coating exhibits a density inthe range of from about 0.95 g/cc to about 0.98 g/cc measured accordingto ASTM D 1505-68; wherein said chromium-containing catalyst comprises achromium oxide; and wherein said yttrium-containing catalyst isrepresented by the formula (Cp₂ YX_(x))_(y).M_(z) L_(n), wherein Cp iscyelopentadienyl or cyclopentadienyl substituted with an alkyl or alkylsilyl radical, X is a halogen, M is an alkali metal, L is a suitableelectron donor ligand, x is 1 or 2, y is 1 or 2, z is 0 or 1, and n is anumber corresponding to the value needed to form a stable complex, withthe proviso that when y is 2, z and n are
 0. 2. A polymer compositepowder comprising a low density ethylene polymer core surrounded by ahigh density ethylene polymer coating;wherein said low density coreexhibits a density in the range of from about 0.88 g/cc to about 0.94g/cc measured according to ASTM D 1505-68, and wherein said high densitycoating exhibits a density in the range of from greater than about 0.94g/cc to about 0.98 g/cc measured according to ASTM D 1505-68.
 3. Apolymer composite powder according to claim 2 wherein said low densitycore exhibits a density in the range of from about 0.90 g/cc to about0.94 g/cc, andwherein said high density coating exhibits a density inthe range of from about 0.95 g/cc to about 0.98 g/cc.
 4. A polymercomposite powder according to claim 2 wherein said low density core ispresent in an amount in the range of from about 10 weight percent toabout 90 weight percent based on the total weight of the polymercomposite powder and said high density coating is present in an amountin the range of from about 90 weight percent to about 10 weight percentbased on the total weight of the polymer composite powder.
 5. A polymercomposite powder according to claim 4 wherein said low density core ispresent in an amount in the range of from about 25 weight percent toabout 90 weight percent based on the total weight of the polymercomposite powder and said high density coating is present in an amountin the range of from about 75 weight percent to about 10 weight percentbased on the total weight of the polymer composite powder.
 6. A polymercomposite powder according to claim 5 wherein said low density core ispresent in an amount in the range of from 50 weight percent to 90 weightpercent based on the total weight of the polymer composite powder andsaid high density coating is present in an amount in the range of from50 weight percent to 10 weight percent based on the total weight of thepolymer composite powder.